
theorem Th721:
Sum digits(3920,10) = 14
proof
  thus Sum digits(3920,10) = Sum <%0,2,9%>+Sum <%3%> by Th720,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%9%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%9%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 2 + 9) + Sum <%3%> by AFINSQ_2:53
  .= 11 + 3 by AFINSQ_2:53
  .= 14;
end;
